The extraction system in which a connecting element in the form of a hose or nozzle is connected to an exhaust-gas source is commonly used. Typical examples are inspection halls, assembly halls, workshops or parking halls for emergency vehicles in which the connecting element is connected to the exhaust pipe of the vehicle. The connecting element is generally designed such that its engagement with the exhaust pipe is released when the vehicle drives off. For safety reasons, the system should be provided with a safety coupling which is intended to disengage if the force between the connecting element and the exhaust-gas source exceeds a certain limit value. Without a safety coupling, the connecting element, which is often formed by a nozzle and associated hose section, can flick back with great force in an uncontrolled manner when the connection to the exhaust pipe comes loose. This can cause significant damage to, for example, vehicle and personnel, but also to, for example, the suspension or securement of the system.
The safety couplings can be based, for example, on friction, in which case two flanged coupling halves are held together by means of a surrounding rubber collar. The functioning is here dependent on the configuration, tolerances and possible deformations, caused by previous disengagements, of the flanges, but also on the material, configuration and condition of the collar.
Another example, which is a combined friction and spring solution, is given in WO 02/42015 A1. The safety coupling which is described therein comprises a first and a second tubular coupling half, each of which comprises a flange portion. One flange portion is straight and the other is conical. A resilient locking ring is arranged to grip over the two flange portions in order to hold together the two coupling halves. The locking ring comprises mutually movable segments, which, with spring arrangements, are resiliently pretensioned into a first position, in which the locking ring straddles the flanges and  holds together the two coupling halves. The segments are displaceable counter to the pretension force into a second position, in which the flange portions and thus the coupling halves can be moved apart. The solution is intrinsically complicated and expensive, at the same time as it is tolerance-sensitive, since it is based on the locking ring sliding over a conical surface of the flange portions during disengagement.
Another principle is given in DE 19622860. The safety coupling which is described therein forms part of an exhaust-gas extraction device, which is equipped with a slotted suction duct on which a trolley can be run. The safety coupling comprises two external hose coupling parts for coupling together two hose ends. Inside the respective hose coupling parts there is a wire coupling part. Each wire coupling part is connected to a wire. In the coupled state, the wire, via the safety coupling, will run between the nozzle and the trolley. The wire coupling is spring-loaded for disengagement should a preset limit value load be exceeded. The technology is relatively complicated and requires some fine-tuning in order to work.
Other solutions employ shearing pins which break if a certain limit value is exceeded. These solutions are awkward to reset after disengagement and further require that the user maintains a spare parts store.
Many known solutions are complicated to reset and, moreover, often require two persons. The latter applies, in particular, to those solutions which are based on friction, since the resetting requires that the friction force required for disengagement has to manually overcome in the resetting. A contributory factor is that the disengagement forces normally range between 250 and 500 N. In addition, many known safety couplings are often also provided with supplementary shock-absorbing rubber covers, which make resettings yet more difficult, since these generally have to be forced into place.